Cultured rat basophilic leukemia (RBL-2H3) cell labeled with myo(2- 3H)inositol for 24 hr showed, upon exposure to cobra cardiotoxin (Maja maja kaauthia), a substantial (5 to 7 fold) increase in the amount of 3H-label in the phosphatidyl inositol monophosphate (PIP) pool. The extent of increase was dependent on concentration and time of exposure to cardiotoxin. The kinetics of redistribution of label within the various inositol phosphate pools suggested that a major action was the stimulation of phosphatidylinositol kinase activity which converts phosphatidylinositol (PI) to PIP. This action was confirmed by assay of the enzyme activity in membrane preparations. Cardiotoxin also caused a time- and concentration- dependent increase in inositol phosphates (4-8%), a slow increase in cytosol Ca2t, secretion of histamine and release of lactate dehydrogenase (LDH) dehydrogenase (LDH) from the 2H3 cells. The release of histamine but not LDH was totally dependent on external calcium. The results suggest that cardiotoxin-stimulated release of histamine from 2H3 cells is through a Ca(2+)-dependent noncytotoxic process. This and previous studies indicate the presence of highly active inositol phospholipid kinases and phosphomonoesterases in membranes of RBL - 2H3 cells.